Single crystal silicon, which is the starting material for most processes for the fabrication of semiconductor electronic components, is commonly prepared with the so-called Czochralski (Cz) process wherein a single seed crystal is immersed into molten silicon and then grown by slow extraction. Molten silicon is contaminated with various impurities, among which is mainly oxygen, during the time it is contained in a quartz crucible. At the temperature of the silicon molten mass, oxygen comes into the crystal lattice until it reaches a concentration determined by the solubility of oxygen in silicon at the temperature of the molten mass and by the actual segregation coefficient of oxygen in the solidified silicon. Such concentrations are greater than the solubility of oxygen in solid silicon at temperatures typical for the processes used to fabricate electronic devices. As the crystal grows from the molten mass and cools, therefore, the solubility of oxygen in it decreases rapidly, whereby in the wafers sliced from the crystal, oxygen is present in supersaturated concentrations.
Thermal treatment cycles typically employed in the fabrication of electronic devices can cause the precipitation of oxygen in silicon wafers which are supersaturated in oxygen. Depending upon their location in the wafer, the precipitates can be harmful or beneficial. Oxygen precipitates located in the active device region of the wafer can impair the operation of the device. Oxygen precipitates located in the bulk of the wafer, however, are capable of trapping undesired metal impurities that may come into contact with the wafer. The use of oxygen precipitates located in the bulk of the wafer to trap metals is commonly referred to as internal or intrinsic gettering (“IG”).
Thermal treatment cycles suitable for achieving internal gettering in a single crystal silicon wafer include rapid thermal anneal (e.g., the Magic Denuded Zone® process by SunEdison Semiconductor, Ltd.) or a long duration anneal in an inert gas ambient atmosphere, such as argon. The short annealing duration in a rapid thermal anneal process is a cost effective solution. However, the precipitate free zone (PFZ, also referred to as denuded zone) depth is typically too deep to getter metallic impurities effectively in the top 100 micrometers of silicon (i.e., the typical amount of silicon left over after back grinding). Conversely, the long duration anneal can achieve both a good PFZ zone (tunable within the top 20 micrometers) and gettering capability. However, the long duration anneal requires annealing time (several hours), which impacts manufacturing cost and output.
Rapid thermal anneal in a nitrogen containing gas ambient atmosphere was developed as an alternative to the Magic Denuded Zone® process and long duration anneal. Rapid thermal anneal in a nitriding ambient atmosphere, e.g., NH3 or N2 gas, achieves strong internal gettering capability with shallow PFZ (Precipitate Free Zone, or Denuded Zone). See, e.g., J Appl Phys, 114, 043520 (2013). It has been previously unrecognized in the art that rapid thermal anneal in a nitrogen gas containing ambient atmosphere may degrade gate oxide integrity yield (GOI).